The invention relates to hydraulic servo valves, and is more particularly concerned with servo valves for controlling high performance, high speed actuators of the type including a hydraulic amplifier first stage and a spool valve second stage. The invention is specifically directed to a servo valve which includes a deceleration control mechanism to prevent high acceleration, i.e., high forces, in the actuator served by the servo valve, in the event of servo failure or actuator failure.
Fluid control valves, to wit, hydraulic servo valves, are well known and many examples exist. One typical servo valve is described in U.S. Pat. No. 3,228,423. These valves are frequently used to control movement of a hydraulic actuator where precision control of movement and rapid response are required. These servo valves have good linearity of response, with output proportional to input over a wide range. In valves of this type, there is a first stage which comprises a so-called torque motor which controls a flapper situated between a pair of fluid jets, which are coupled to amplifier ports. Supply pressure and return or drain are connected to this first stage, and the torque motor controls the flapper such that there are differential pressures applied to the amplifier ports which are in proportion to the signal applied to the armature of the torque motor.
A spool type second stage comprises a valve body with suitable channeling and porting, a cylindrical sleeve or bushing with precision-cut ports and openings and a spool which travels back and forth in the bushing. The spool has lands and grooves on the cylindrical face so that the lands and the ports in the bushing define precision variable orifices that deliver flows in proportion to the amplifier outputs, to first and second control ports. The outputs at the amplifier ports are applied to the first and second ends of the spool to urge it right or left of a center or null position. In the null position the output pressures at the control ports are in balance.
An arm mechanically couples the spool to the flapper, and provides mechanical feedback to the first stage.
There are numerous variations in the basic design of the hydraulic servo valve, and some of these appear in U.S. Pats. Nos. 3,221,760; 4,617,966; and 4,456,031. A spool and bushing for a second stage is described in U.S. Pat. No. 4,337,797.
In the case of high-speed, high-performance actuators, it is desired to include a mechanism to limit acceleration of the controlled hydraulic actuator in the event of failure of the hydraulic amplifier stage or of control circuitry upstream of it. To accomplish this it is necessary to override the first stage and urge the spool of the second stage quickly, but smoothly, to a set point near its null position. The object here is to decelerate the actuator travel within a small portion of its throw or stroke length, but to do so in a fashion to limit acceleration or deceleration to an acceptable level, ideally to less than 0.4 g.